In order to better understand the concept of stress it is required to first have some basic knowledge about the interrelationship of the three systems that function parallel to maintain a person—these are the physiological, psychological and social systems. A common trait of all of them is that they develop over time and undergo changes (Trumbull & Appley, 1986). The physiological system has subsystems which functioning depends on biochem- ical and neurological processes. The efficiency of the physiological subsystems is evident by the subjective state of health of one’s body, the availability to perform work and create work products, and by waste products. The psychological system is also characterized by subsystems, for example memory, cognition, perception, emotional experience and regulation, etc. The psychological subsystems are generally influenced by the functioning of some or all of the physiological subsystems and their processing is assessed through characteristic personality traits, temperament types, behavioral patterns, learning out- comes, etc. The social system includes different social and moral norms and values, cultural attitudes and differences, social in-group belongingness, etc. The role of the social system is to provide support for the other two major systems.

The dynamic process model of stress of Trumbull and Appley (1986) explains stress as a process by which functioning of one or more of the systems described above is damaged due to a discrepancy between a stressor and the carrying capacity of one or all of the systems of a person. The stressor is a demand that can be real or perceived and ‘may arise from an eventful, chronic or cumulative’ (Trumbull & Appley, 1986, p. 34) events and from ‘a change within the systems’ (Trumbull & Appley, 1986, p. 34). Stress results when the distance between demands and carrying capacity increases beyond given optimal level that is tolerable for the organism, e.g. even if demands increase be- yond normal level stress would not be experienced when carrying capacity is still able to meet the demands and the distance between them remains optimal.